[time-nuts] Insulation - was: OCXO and Aging

Bob Smither smither at c-c-i.com
Thu Sep 15 04:36:09 UTC 2011

Will Matney wrote:
> Paul,
> The info they show is generally from the manual or paperwork that comes with
> the OCXO when new.
> If they are used OCXO's, then they will be more stable over time than a new
> one, off the shelf, as far as drift, or they should be. The OCXO acts
> similar to any heated device, and resistors function similar to this also.
> When first used, they will drift from spec more than they do after they have
> ran for so long in time. The factory generally does a burn in to stabilize
> them, but it still takes some time after that to completely settle down to
> where calibration isn't needed as often. There's generally a curve for each
> model that will show the drift over time.
> The main thing in using these, even though they're supposed to be insulated,
> is to mount them where temperature fluctuations are minimal. Also, some are
> better insulated than others, like the HP units, which are encased in foam
> inside a can.


I scanned the archives for this topic, so if I missed it and it has 
already been beat to death, please point me to any previous discussions.

What is the list's opinion about putting OCXOs in an insulated 
enclosure?  That should reduce the effect of drafts in the lab.  I have 
some insulin shipping boxes and put an OCXO in one for a few days.  The 
OCXO is an C-MAC STP2145A.  I measured the current draw of the OCXO and 
the metal case temperature with the lid of the insulated box on and off:

Lid off: Is ~ 163 mA, Tc ~ 105F
Lid on : Is ~ 130 mA, Tc ~ 118F

I assume the oven operates at the same temperature in both cases. 
Clearly there is less power needed as less heat is escaping.  It looks 
like the other circuit elements (outside the oven) are operating at 
higher temperatures - likely not a good thing.

My measuring capability is limited.  Within the precision (~10E-8) that 
I have I could not see any difference in the oscillator frequency.


Bob Smither
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